The present invention relates to brake control system and method for motor vehicles, which allow antilock (ABS) controlled braking for preventing wheels from being locked by controlling the brake-fluid pressure. Particularly, the present invention relates to the brake control system and method that ensure correct ABS control even when a wheel-velocity sensor falls abnormal.
In brake control systems for allowing ABS controlled braking, a pseudo vehicle-body velocity is estimated in accordance with a velocity of each wheel, and a reduction, a retention and an increase in the brake-fluid pressure are carried out in accordance with the pseudo body velocity and the wheel velocity to prevent wheels from being locked during braking, obtaining both secured steerability and running stability and shortened braking distance.
In the systems, the detection accuracy of the wheel velocity is important for ABS control. Thus, if a wheel-velocity sensor falls abnormal, correct ABS control is difficult to carry out. In view of this, brake control systems are proposed wherein an anomaly of the wheel-velocity sensor is detected to carry out ABS control taking account of this anomaly.
One of such brake control systems is disclosed in JP-A 8-332941. The system includes anomaly detecting means for detecting an anomaly of a velocity of a wheel and for making, when detecting its anomaly, estimated body-velocity calculating means calculate an estimated body velocity in accordance with velocities of the wheels excluding an abnormal wheel. The anomaly detecting means carry out determination such that when the wheel velocity calculated in this control cycle is greater than that one calculated in the preceding control cycle by a predetermined value, i.e. the fluctuation rate of the wheel velocity is greater than a predetermined value, the wheel velocity is abnormal.
Thus, with the system, even if output of the wheel-velocity sensor is greater due to noise superimposed thereon, the estimated body velocity is not greater than the actual body velocity, preventing needless ABS control that results in a needless reduction in the brake-fluid pressure. Moreover, even when the wheel velocity returns abruptly after occurrence of a great wheel skid, erroneous determination is not carried out which the wheel velocity is abnormal, preventing needless ABS control that results in a needless reduction in the brake-fluid pressure.
JP-A 7-329759 discloses detection of noise superimposed on the wheel velocity in accordance with the frequency.
However, due to their structure that an anomaly of the wheel velocity is determined in accordance with the fluctuation rate or the frequency of noise superimposed on the wheel velocity, the above brake control systems cannot detect an anomaly when noise is superimposed having the fluctuation rate smaller than a predetermined value or the frequency out of a predetermined frequency range, which may result in the following problem:
During ABS control, noise with the relatively low fluctuation rate and the low frequency is superimposed on the wheel velocity, the pseudo body velocity increases with a rise in the wheel velocity with noise superimposed, and a reduced-pressure threshold value xcex obtained from a pseudo body velocity Vi increases as well. Thus, a velocity VwB of the three normal wheels is smaller than the reduced-pressure threshold value xcex, so that a pressure reduction is carried out with regard to the three normal wheels or the four wheels dependent on the form of ABS control. In the latter case, even with a pressure reduction of each normal wheel, the wheel velocity VwB returns to a real wheel velocity, but not to the pseudo body velocity Vi with noise superimposed, this pressure reduction may be carried out endlessly, resulting in insufficient achievement of deceleration.
It is, therefore, an object of the present invention to provide brake control system and method for allowing ABS controlled braking, which contribute to an improvement in the control quality.
One aspect of the present invention lies in providing a motor vehicle, comprising:
wheels;
wheel cylinders arranged with said wheels;
a brake unit that controls pressures within said wheel cylinders;
wheel-velocity sensors that detect velocities of said wheels; and
a control unit connected to said brake unit and said wheel-velocity sensors, said control unit being so constructed as to:
obtain a pseudo body velocity in accordance with outputs of said wheel-velocity sensors,
control said brake unit in accordance with a difference between each of said wheel velocities and said pseudo body velocity to reduce said pressures for ABS control,
determine that one of said outputs of said wheel-velocity sensors is abnormal when a pressure-reduction time measured during ABS control exceeds a predetermined value, and
form said pseudo body velocity, if an anomaly of said one of said outputs of said wheel-velocity sensors is determined, in accordance with said outputs of said wheel-velocity sensors excluding said abnormal wheel-velocity sensor.
Another aspect of the present invention lies in providing a method of controlling a motor vehicle provided with wheels, wheel cylinders, a brake unit for controlling pressures within the wheel cylinders, and wheel-velocity sensors for detecting velocities of the wheels, the method comprising the steps of:
obtaining a pseudo body velocity in accordance with outputs of the wheel-velocity sensors;
controlling the brake unit in accordance with a difference between each of said wheel velocities and said pseudo body velocity to reduce the pressures for ABS control;
determining that one of said outputs of the wheel-velocity sensors is abnormal when a pressure-reduction time measured during ABS control exceeds a predetermined value; and
forming said pseudo body velocity, if an anomaly of said one of said outputs of the wheel-velocity sensors is determined, in accordance with said outputs of the wheel-velocity sensors excluding the abnormal wheel-velocity sensor.
Still another aspect of the present invention lies in providing a motor vehicle, comprising:
wheels;
wheel cylinders arranged with said wheels;
a brake unit that controls pressures within said wheel cylinders;
wheel-velocity sensors that detect velocities of said wheels;
means for obtaining a pseudo body velocity in accordance with outputs of said wheel-velocity sensors;
means for controlling said brake unit in accordance with a difference between each of said wheel velocities and said pseudo body velocity to reduce said pressures for ABS control;
means for determining that one of said outputs of said wheel-velocity sensors is abnormal when a pressure-reduction time measured during ABS control exceeds a predetermined value; and
means for forming said pseudo body velocity, if an anomaly of said one of said outputs of said wheel-velocity sensors is determined, in accordance with said outputs of said wheel-velocity sensors excluding said abnormal wheel-velocity sensor.